Digital video capabilities can be incorporated into a wide range of devices, including digital televisions, digital direct broadcast systems, wireless broadcast systems, personal digital assistants (PDAs), laptop or desktop computers, digital cameras, digital recording devices, video gaming devices, video game consoles, tablet computers, cellular or satellite radio telephones, and the like. Digital video devices implement video compression techniques, such as MPEG-2, MPEG-4, On2 VP6/VP7/VP8 coding, or ITU-T H.264/MPEG-4, Part 10, Advanced Video Coding (AVC), to transmit and receive digital video information more efficiently. Video compression techniques perform spatial prediction and temporal prediction to reduce or remove redundancy inherent in video sequences.
In video coding, video compression generally includes spatial prediction, motion estimation, and motion compensation. Intra-coding relies on spatial prediction to reduce or remove spatial redundancy between video blocks within a given coded unit, which may comprise a video frame, a slice of a video frame, a group of pictures, or another defined unit of video. Inter-coding relies on temporal prediction to reduce or remove temporal redundancy between video blocks of successive coded units of a video sequence. For intra-coding, a video encoder performs spatial prediction to compress data based on other data within the same coded unit. For inter-coding, the video encoder performs motion estimation and motion compensation to track the movement of corresponding video blocks between two or more adjacent coded units. Motion estimation generates motion vectors, which indicate the displacement of video blocks relative to corresponding prediction video blocks in one or more reference frames. Motion compensation uses the motion vectors to generate the prediction videos block from the reference frames. After motion compensation, a given residual video block is formed by subtracting the prediction video block from the original video block to be coded.
A coded video block may be represented by prediction information that can be used to create or identify a predictive block and a residual block of data indicative of differences between the block being coded and the predictive block. In the case of inter-coding, one or more motion vectors are used to identify the predictive block of data, while in the case of intra-coding, the prediction mode can be used to generate the predictive block. Both intra-coding and inter-coding may define several different prediction modes, which may define different block sizes and/or prediction techniques used in the coding. Additional types of syntax elements may also be included as part of encoded video data in order to control or define the coding techniques or parameters used in the coding process.
After block-based prediction coding, the video encoder may apply transform, quantization, and entropy coding processes to further reduce the bit rate associated with communication of a residual block. Transform techniques may comprise discrete cosine transforms or conceptually similar processes. Alternatively, wavelet transforms, integer transforms, or other types of transforms may be used. In a discrete cosine transform (DCT) process, as an example, the transform process converts a set of pixel values into transform coefficients, which may represent the energy of the pixel values in the frequency domain. Quantization is applied to the transform coefficients, and generally involves a process that limits the number of bits associated with any given transform coefficient. Entropy coding comprises one or more processes that collectively compress a sequence of quantized transform coefficients. Examples of entropy coding processes include arithmetic coding, content adaptive variable length coding (CAVLC), and context adaptive binary arithmetic coding (CABAC).
Regardless of the type of entropy coding used in the encoding process, a video decoder may perform inverse entropy coding operations to reconstruct the coefficients. The video decoder then inverse quantizes and inverse transforms the coefficients. The video decoder may decode the video information based on the motion information and residual information associated with video blocks in order to generate a decoded sequence of video information.